One of the major goals of plant biotechnology is the generation of plants with advantageous novel properties, for example, to increase agricultural productivity, to increase quality in the case of foodstuffs, or to produce specific chemicals or pharmaceuticals. The plant's natural defense mechanisms against pathogens are frequently insufficient. Fungal disease alone results in annual yield loses of many billions of US dollars. The introduction of foreign genes from plants, animals or microbial sources can increase the defense. Examples are the protection of tobacco against feeding damage by insects by expressing Bacillus thuringiensis endotoxins under the control of the 35S CaMV promoter or the protection of tobacco against fungal infection by expressing a bean chitinase under the control of the CaMV promoter. However, most of the approaches described only offer resistance to a single pathogen or a narrow spectrum of pathogens.
A large group of biotrophic plant pathogens of enormous agro-economical importance are nematodes. Nematodes are microscopic roundworms that feed on the roots, leaves and stems of more than 2,000 row crops, vegetables, fruits, and ornamental plants, causing an estimated $100 billion crop loss worldwide. A variety of parasitic nematode species infect crop plants, including root-knot nematodes (RKN), cyst- and lesion-forming nematodes. Root-knot nematodes, which are characterized by causing root gall formation at feeding sites, have a relatively broad host range and are therefore pathogenic on a large number of crop species. The cyst- and lesion-forming nematode species have a more limited host range, but still cause considerable losses in susceptible crops.
Pathogenic nematodes are present throughout the United States, with the greatest concentrations occurring in the warm, humid regions of the South and West and in sandy soils. Soybean cyst nematode (Heterodera glycines), the most serious pest of soybean plants, was first discovered in the United States in North Carolina in 1954. Some areas are so heavily infested by soybean cyst nematode (SCN) that soybean production is no longer economically possible without control measures. Although soybean is the major economic crop attacked by SCN, SCN parasitizes some fifty hosts in total, including field crops, vegetables, ornamentals, and weeds.
Signs of nematode damage include stunting and yellowing of leaves, and wilting of the plants during hot periods. However, nematode infestation can cause significant yield losses without any obvious above-ground disease symptoms. The primary causes of yield reduction are due to root damage underground. Roots infected by SCN are dwarfed or stunted. Nematode infestation also can decrease the number of nitrogen-fixing nodules on the roots, and may make the roots more susceptible to attacks by other soil-borne plant pathogens.
The nematode life cycle has three major stages: egg, juvenile, and adult. The life cycle varies between species of nematodes. For example, the SCN life cycle can usually be completed in 24 to 30 days under optimum conditions whereas other species can take as long as a year, or longer, to complete the life cycle. When temperature and moisture levels become favorable in the spring, worm-shaped juveniles hatch from eggs in the soil. Only nematodes in the juvenile developmental stage are capable of infecting soybean roots.
The life cycle of SCN has been the subject of many studies, and as such are a useful example for understanding the nematode life cycle. After penetrating soybean roots, SCN juveniles move through the root until they contact vascular tissue, at which time they stop migrating and begin to feed. With a stylet, the nematode injects secretions that modify certain root cells and transform them into specialized feeding sites. The root cells are morphologically transformed into large multinucleate syncytia (or giant cells in the case of RKN), which are used as a source of nutrients for the nematodes. The actively feeding nematodes thus steal essential nutrients from the plant resulting in yield loss. As female nematodes feed, they swell and eventually become so large that their bodies break through the root tissue and are exposed on the surface of the root.
After a period of feeding, male SCN nematodes, which are not swollen as adults, migrate out of the root into the soil and fertilize the enlarged adult females. The males then die, while the females remain attached to the root system and continue to feed. The eggs in the swollen females begin developing, initially in a mass or egg sac outside the body, and then later within the nematode body cavity. Eventually the entire adult female body cavity is filled with eggs, and the nematode dies. It is the egg-filled body of the dead female that is referred to as the cyst. Cysts eventually dislodge and are found free in the soil. The walls of the cyst become very tough, providing excellent protection for the approximately 200 to 400 eggs contained within. SCN eggs survive within the cyst until proper hatching conditions occur. Although many of the eggs may hatch within the first year, many also will survive within the protective cysts for several years.
A nematode can move through the soil only a few inches per year on its own power. However, nematode infestation can be spread substantial distances in a variety of ways. Anything that can move infested soil is capable of spreading the infestation, including farm machinery, vehicles and tools, wind, water, animals, and farm workers. Seed sized particles of soil often contaminate harvested seed. Consequently, nematode infestation can be spread when contaminated seed from infested fields is planted in non-infested fields. There is even evidence that certain nematode species can be spread by birds. Only some of these causes can be prevented.
Traditional practices for managing nematode infestation include: maintaining proper soil nutrients and soil pH levels in nematode-infested land; controlling other plant diseases, as well as insect and weed pests; using sanitation practices such as plowing, planting, and cultivating of nematode-infested fields only after working non-infested fields; cleaning equipment thoroughly with high pressure water or steam after working in infested fields; not using seed grown on infested land for planting non-infested fields unless the seed has been properly cleaned; rotating infested fields and alternating host crops with non-host crops; using nematicides; and planting resistant plant varieties.
Methods have been proposed for the genetic transformation of plants in order to confer increased resistance to plant parasitic nematodes. U.S. Pat. Nos. 5,589,622 and 5,824,876 are directed to the identification of plant genes expressed specifically in or adjacent to the feeding site of the plant after attachment by the nematode. U.S. Pat. Nos. 5,589,622 and 5,824,876 disclose eight promoters isolated from potato roots infected with Globodera rostochiensis: no nematode-inducible promoters from other plant species are disclosed. These promoters are purported to be useful to direct the specific expression of toxic proteins or enzymes, or the expression of antisense RNA to a target gene or to general cellular genes.
U.S. Pat. No. 5,023,179 discloses a promoter enhancer element designated ASF-1, isolated from the CaMV promoter, which is purported to enhance plant gene expression in roots.
U.S. Pat. No. 5,750,386 discloses a deletion fragment of the RB7 root specific promoter of Nicotiana tabacum, which is purported to be nematode-responsive.
U.S. Pat. No. 5,837,876 discloses a root cortex specific gene promoter isolated from tobacco and designated TobRD2.
U.S. Pat. No. 5,866,777 discloses a two-gene approach to retarding formation of a nematode feeding structure. The first gene, barnase, is under control of a promoter that drives expression at least in the feeding structure. The second gene, barstar, is under control of a promoter that drives expression in all of the plant's cells except the feeding structure. Feeding site-specific promoters disclosed in U.S. Pat. No. 5,866,777 include truncated versions of the Δ0.3TobRB7 and rolC promoters.
U.S. Pat. No. 5,955,646 discloses chimeric regulatory regions based on promoters derived from the mannopine synthase and octopine synthase genes of Agrobacterium tumefaciens, which are purported to be nematode-inducible.
U.S. Pat. No. 6,005,092 discloses the N. tabacum endo-1,4-β-glucanase (Ntce17) promoter.
U.S. Pat. Nos. 6,262,344 and 6,395,963 disclose promoters isolated from Arabidopsis thaliana, which are purported to be nematode-inducible.
U.S. Pat. No. 6,448,471 discloses a promoter from A. thaliana, which is specific for nematode feeding sites.
U.S. Pat. No. 6,703,541 discloses cloning and isolation of maize peroxidase P7X gene and its promoter, the promoter is purported to be nematode inducible.
U.S. Pat. No. 6,593,513 discloses transformation of plants with bamase under control of the promoter of the A. thaliana endo-1,4-β-glucanase gene (cel1) to produce plants capable of disrupting nematode attack.
U.S. Pat. No. 6,906,241 discloses use of the Ntce17 promoter in combination with a heterologous nucleic acid that encodes a nematocidal or insecticidal protein.
U.S. Pat. No. 7,078,589 discloses cloning and isolation of the soybean Pyk20 gene and promoter, which are purported to be induced by SCN infection and to show strong activity in vascular tissues.
U.S. Patent Application Publication No. 2003/0167507 discloses the promoter of soybean isoflavone synthase I, which is purported to be root specific and inducible in vegetative tissue by parasite attack.
U.S. Patent Application Publication No. 2004/0078841 discloses promoter regions of the TUB-1, RPL16A, and ARSK1 promoters of Arabidopsis thaliana and the PSMTA promoter from Pisum sativum, all of which are purported to be root-specific.
U.S. Patent Application Publication No. 2004/0029167 discloses a promoter sequence of a class II caffeic acid O-methyltransferase gene from tobacco, which is purported to be inducible in response to mechanical or chemical injury or to aggression by a pathogenic agent.
U.S. Patent Application Publication No. 2005/0262585 discloses a promoter from soybean phosphoribosylformylglycinamidine ribonucleotide synthase and deletion fragments thereof, which are purported to be responsive to nematode infection.
WO 94/10320 discloses the Δ0.3TobRB7 promoter fragment from tobacco and its use with a variety of genes for nematode feeding cell-specific expression.
WO 03/033651 discloses synthetic nematode-regulated promoter sequences designated SCP1, UCP3, and SUP.
WO 2004/029222 and its US counterpart U.S. Patent Application Publication No. 2005/0070697 disclose regulatory regions from the soybean adenosine-5′-phosphate deaminase and inositol-5-phosphatase genes, for use in improving nematode resistance in plants.
None of the above-mentioned root- or feeding-site specific promoters are currently in use in commercial seed containing an anti-pathogen transgene. Although the need for such products has long been acknowledged, no one has thus far succeeded in developing pathogen-resistant plants through recombinant DNA technology. A need continues to exist for root-specific and/or nematode feeding site preferred promoters to combine with transgenes encoding agents toxic to plant parasitic pathogens.